1. Field of the Invention
The present invention relates to a semiconductor optical modulator, which is an important element in an optical communication or an optical information processing system, and, particularly, to a semiconductor electroabsorption optical modulator, in which electroabsorption rate is changed by applying an electric field thereto, and a fabrication method for fabricating the same semiconductor electroabsorption optical modulator.
2. Description of the Prior Art
In an optical modulator, which is a key device in an optical communication system whose bit rate and communication distance are increased more and more recently, specifically, an electroabsorption type optical modulator, which utilizes a phenomenon that the optical absorption edge of a semiconductor material is shifted toward a long wavelength side upon application of an electric field, a multiple quantum well (MQW) layer is usually used as an optical absorption layer, as shown in FIG. 1.
The optical modulator shown in FIG. 1 has a striped multi-layered structure including an n type InP buffer layer 2, an n type InGaAsP optical guide layer 3, an n type InP clad layer 4, a non-doped MQW optical absorption layer 5 formed by alternately laminating InGaAsP well layers and InGaAsP barrier layers, a p type InGaAsP clad layer 6 and a p type InP clad layer 8, which are formed on an n type InP substrate in the order.
Semi-insulating InP burying layers 9 are formed on both sides of the multi-layered structure and a p+InP clad layer 10 and a p+InGaAs contact layer 11 are further formed on the semi-insulating InP burying layer 9 and the p type InP clad layer 8.
In the optical modulator having such structure, optical ON/OFF ratio (extinction ratio) per unit voltage application is large since the MQW layer is used as the optical absorption layer. However, there may be diffusion of p type impurity from the p+InGaAs contact layer 11 and the p+InP clad layer 10 on the optical absorption layer 5 to the InGaAsP clad layer 6 adjacent to the optical absorption layer 5, during a fabrication thereof. Therefore, strength of electric field applied to the optical absorption layer 5 becomes large in a portion of the optical absorption layer 5, so that electric field strength distribution in the optical absorption layer becomes uneven. Since the degree of absorption in the electroabsorption type optical modulator depends on square of electric field strength, the extinction ratio characteristics with respect to applied voltage becomes non-linear as shown by a curve 52 in FIG. 5 when there is an unevenness of electric field strength within the optical absorption layer.
Therefore, when a high speed modulation higher than 2.5Gb/s is performed, cross points of eye pattern, which is a modulated waveform, are lowered as shown in FIG. 6B, so that there is a problem that an acceptable eye opening can not be obtained.
An object of the present invention is to provide an electroabsorption type optical modulator capable of preventing impurity from diffusing to a vicinity of an optical absorption layer thereof and maintaining cross points in an ideal state and a fabrication method for fabricating the same electroabsorption type optical modulator.
An electroabsorption optical modulator according to the present invention, which includes a striped waveguide constructed with a multi-layered structure, in which an optical absorption layer is sandwiched between clad layers having band gap energy larger than that of the optical absorption layer, formed on a semiconductor substrate, so that absorption of light incident on one edge of the optical absorption layer is changed by changing strength of electric field applied to the optical absorption layer, is featured by that, between a first upper clad layer adjacent to an upper surface of the optical absorption layer and a second upper clad layer, an impurity diffusion preventing layer for preventing p type impurity from diffusing from the second upper clad layer and a semiconductor layer provided above the second upper clad layer to the first upper clad layer is provided.
When the impurity diffusion preventing layer is a semiconductor layer with carrier concentration lower than that of the second upper clad layer and p type impurity diffusion speed is lower than that of the first upper clad layer, the impurity diffusion preventing effect thereof becomes considerable. Furthermore, it is preferable that the waveguide has a multi-layered structure constructed by laminating an optical guide layer of a first conductivity type, a lower clad layer of the first conductivity type having band gap energy larger than that of the optical guide layer, an MQW optical absorption layer having band gap energy smaller than that of the optical guide layer, a first upper clad layer of a second conductivity type having band gap energy smaller than that of the lower clad layer, an impurity diffusion preventing layer of the second conductivity type having carrier concentration lower than that of the first clad layer and a second upper clad layer of the second conductivity type having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion preventing layer, in the order.
Moreover, when the striped waveguide is buried by high resistance semiconductor layers provided on both sides thereof, light confinement efficiency thereof becomes excellent, resulting in high modulation efficiency.
Furthermore, in the electroabsorption type optical modulator according to the present invention, it is preferable to that the semiconductor substrate, the lower clad layer, the impurity diffusion preventing layer and the second upper clad layer are formed of InP, the optical guide layer is formed of InGaAsP, the optical absorption layer is of InGaAsP MQW and the first upper clad layer is formed of InGaAsP.
The fabrication method for fabricating the electroabsorption type optical modulator, according to the present invention, is featured by comprising the step of forming, on a semiconductor substrate of a first conductivity type, a multi-layered structure by sequentially forming at least a lower clad layer of the first conductivity type, a non-doped optical absorption layer having band gap energy smaller than that of the lower clad layer, a first upper clad layer having band gap energy larger than that of the optical absorption layer and smaller than that of the lower clad layer, an impurity diffusion preventing layer of a second conductivity type having band gap energy larger than that of the first clad layer and carrier concentration lower than that of the first upper clad layer and a second upper clad layer having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion preventing layer, the step of forming a waveguide having a striped multi-layered structure by etching the multi-layered structure, the step of forming a burying layer of high resistance semiconductor on both sides of the waveguide and the step of forming, on at least the waveguide, a semiconductor layer of the second conductivity type having carrier concentration higher than that of the second upper clad layer.
A second fabrication method for fabricating the electroabsorption type optical modulator according to the present invention is featured by comprising the steps of forming a mask of dielectric material having a striped opening on a semiconductor substrate of a first conductivity type, forming a waveguide having a striped multi-layered structure by sequentially forming, on the opening of the mask by selective growth, at least a lower clad layer of the first conductivity type, a non-doped optical absorption layer having band gap energy smaller than that of the lower clad layer, a first upper clad layer having band gap energy larger than that of the optical absorption layer and smaller than that of the lower clad layer, an impurity diffusion preventing layer of a second conductivity type having band gap energy larger than that of the first clad layer and carrier concentration lower than that of the first upper clad layer and a second upper clad layer having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion preventing layer, forming a burying layer of high resistance semiconductor on both sides of the waveguide by selective growth and forming, on at least the waveguide, a semiconductor layer of the second conductivity type having carrier concentration higher than that of the second upper clad layer.
In the above two fabrication methods, the waveguide preferably includes an optical guide layer of the first conductivity type having band gap energy smaller than that of the lower clad layer of the first conductivity type and larger than that of the optical absorption layer below the lower clad layer.
Moreover, it is preferable that the semiconductor substrate, the lower clad layer, the impurity diffusion preventing layer and the second upper clad layer are formed of InP, the optical guide layer is formed of InGaAsP, the optical absorption layer is of InGaAsP MQW and the first upper clad layer is formed of InGaAsP.
Furthermore, in view of the impurity diffusion prevention, it is preferable to reduce the growing time or the growing temperature of the semiconductor layer formed after the waveguide is formed, that is, to reduce the growing time or the growing temperature of the burying layer and the semiconductor layer formed on the waveguide.